an Explanation of Hall's Phenomenon. 251 



across the gold-leaf is opposite to that in which the gold-leaf 

 itself would move across the lines of force if it were free to do 

 so. If the two points to which the galvanometer is connected 

 are joined by an equipotential line across the gold-leaf, the 

 effect caused by magnetization may be expressed by saying 

 that the equipotential line is rotated in a direction opposite to 

 that of the current circulating in the coils of the electro- 

 magnet. 



A second paper was published in the ' Philosophical Maga- 

 zine' for November 1880, in which were described experiments 

 made with other metals in addition to gold. With silver, 

 nickel, tin, and platinum the direction of the transverse effect 

 was the same as with gold, though it differed in degree in the 

 case of every metal. But with iron the direction of the trans- 

 verse current was found to be reversed; a fact which Mr. Hall 

 was naturally inclined to attribute to the magnetic properties 

 of that metal, though he was at a loss to account for the differ- 

 ence between its behaviour and that of nickel, which is also 

 strongly magnetic. In iron, therefore, the equipotential lines 

 are rotated in the same direction as that of the current circu- 

 lating in the coils of the electromagnet. 



In a third paper, published in the i Philosophical Magazine ' 

 for September 1881, the author says that the direction of the 

 transverse electromotive force in the magnetic metal cobalt 



E' . 



was the same as in iron. In this paper it is stated that ^ is 



probably a constant for any given metal ; E' being the trans- 

 verse electromotive force per centimetre of the width of the 

 strip, and V the direct current divided by the section of the 



W 



conductor. This quantity ^ is called, in accordance with a 



suggestion of Dr. Hopkinson, the " rotational coefficient" of 

 the metal. The direction of the transverse effect or sign of the 

 rotational coefficient is in iron and metals which behave like 

 iron called +, because its direction is that which the con- 

 ductor itself bearing the current would follow if free to move 

 across the lines of magnetic force. In gold and similar metals 

 the sign of the rotational coefficient is called — . 



A fourth paper appeared in the i Philosophical Magazine ' 

 for May 1883, entitled u Rotational Coefficients of various 

 Metals/'' in which it is stated that aluminium, copper, and 

 brass behave in the same manner as gold, that zinc behaves 

 like iron, and that with lead no transverse effect whatever is 

 produced. 



Collecting Mr, Hall's various results, we find that, of the 



S2 



